In general, a hydraulic construction machine is equipped with a hydraulic actuator such as a hydraulic cylinder driving a front work device mounted thereon, an operation device operated by the operator, a hydraulic pump adjusting the delivery flow rate in accordance with the operation amount of the operation device, and a control valve driving a built-in directional control valve with an operation pilot pressure in accordance with the operation amount of the operation device to control the flow rate and direction of the hydraulic fluid supplied from a hydraulic pump to the hydraulic actuator.
When the hydraulic construction machine performs an operation such as excavating, there is generated inside the hydraulic actuator driving the front work device a load pressure in accordance with the excavating reaction force (excavating load), and the delivery pressure of the hydraulic pump is a value obtained by adding together this load pressure and the pressure loss of the hydraulic fluid line. In view of this, the hydraulic construction machine adopts a pump horsepower control in which as the delivery pressure of the hydraulic pump increases, the capacity of the hydraulic pump (delivery flow rate) is reduced to lower the horsepower of the hydraulic pump. The pump horsepower control suppresses deterioration in efficiency due, for example, to the application of an excessive load to the engine driving the hydraulic pump, an excessive increase in the delivery pressure of the hydraulic pump, and an increase in leak flow rate.
In connection with this hydraulic construction machine, there exists a construction machine locus control system converging the front device distal end to a target locus via a satisfactory path always matched with the human feeling independently of the operation amount of the operator (see, for example, Patent Document 1). This locus control system computes the position and attitude of the front device based on a signal from an angle sensor, and computes a target speed vector of the front device based on a signal from an operation lever device. The target speed vector is corrected so as to be directed to a point advanced forwards in the excavation progressing direction by a predetermined distance from a point in the target locus that is at a minimum distance from the front device distal end, and there is computed a target pilot pressure for driving a hydraulic control valve in correspondence with the corrected target speed vector. A proportional solenoid valve is controlled so as to generate the computed target pilot pressure.
Further, there exists a work device control system for a construction machine that aims to improve the position follow-up property of a work device operation cylinder and to secure predetermined finish accuracy even if the excavating load increases during a horizontally leveling operation or a slope face forming operation (see, for example, Patent Document 2). This work device control system constitutes a position follow-up feedback control system controlling a pilot pressure by a solenoid proportional valve so as to eliminate an error between the target position and target speed of each cylinder based on a signal from an operation lever and the actual position and speed of each cylinder based on information obtained from an angle sensor, and adjusts to increase the feedback gain and the feed forward gain by a lookup table in accordance with an increase in the cylinder load pressure.